Measurement of vocal tract area functions from MRI data requires a technique for tooth visualization because the teeth are as transparent as air in the images. In this article, a new method is proposed to accurately superimpose the teeth onto MRI volume data. Upper and lower tooth images with the surrounding bony structure are obtained by scanning a subject holding a contrast medium in the oral cavity. They are superimposed onto the target volume data via a three-dimensional transformation using landmarks sampled from the tooth images and target MRI data. The accuracy of the dental image superimposition is ensured by the minimization of the error volume, which is the mismatch volume of the dental image overlapping the surrounding soft tissue. The method is evaluated using five operators sampling the landmarks. Results show that the error volume is significantly reduced to a nearly constant value regardless of the operator's skill.

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One issue in externally triggered cine-magnetic resonance imaging (EC-MRI) for the dynamic observation of speech organs is motion artifact in the phase-encoding direction caused by unstable repetitions of speech during data acquisition. We propose a technique to reduce such artifact by rearranging the k-space data used to reconstruct MR images based on the analysis of recorded speech sounds.We recorded the subject's speech sounds during EC-MRI and used post hoc acoustical processing to reduce scanning noise and detect the onset of each utterance based on analysis of the recorded sounds. We selected each line of k-space from several data acquisition sessions and rearranged them to reconstruct a new series of dynamic MR images according to the analyzed time of utterance onset. Comparative evaluation showed signiˆcant reduction in motion artifact signal in the dynamic MR images reconstructed by the proposed method. The quality of the reconstructed images was su‹cient to observe the dynamic aspects of speech production mechanisms.

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The accuracy of vocal tract transfer functions estimated by Finite Element Method (FEM) analyses was evaluated to find relationships between structural characteristics of vocal tracts and their acoustical characteristics. Three-dimensional vocal tract models were constructed from MR Images of four speakers (two normal persons and two patients after tongue and mouth floor resection), uttering the Japanese vowel /i/. Vocal tract transfer functions of the models were estimated by FEM and one-dimensional models, and peak positions of these functions were compared with formant frequencies of these speaker’s speech waves. The patient speech waves had a peculiar formant at around 1500 Hz. The peak positions of the patients estimated by FEM were similar to the formant frequencies of the patients, especially the formant at 1500 Hz. However, the transfer functions by the one-dimensional models had no peak at around 1500 Hz and the number of peaks was fewer than that by FEM. Since the patient vocal tract shapes were not symmetrical and bent right and left, sound pressure distributions estimated by FEM showed that the waves in the patient vocal tracts cannot be assumed as plane waves. Then, the one-dimensional models cannot describe acoustical characteristics of the patient vocal tracts.